1. FIELD OF THE INVENTION
The present invention relates to a molten carbonate type fuel cell or battery, more particularly it relates to an electrode for use in the molten carbonate type fuel cell or battery and a method of preparing the electrode. This electrode is prevented from being plastically deformed and improved in reducing contact resistance between the electrodes and an electrolyte plate.
2. RELATED ART
Generally, the molten carbonate type fuel cell comprises a single unit cell comprising two electrodes as anode 1 and cathode 2, an electrolyte plate 3 and separators 4, as shown in FIG. 1. Furthermore, collectors may be provided between the separators and the anode and/or cathode. A plurality of the single unit cells are stacked to form a battery. In this fuel cell, an alkali metal carbonate such as lithium carbonate and potassium carbonate is used as an electrolyte, a fuel gas containing hydrogen is allowed to pass through the channels of the separator facing the anode and, on the other hand, a mixed gas of air and oxygen or carbon monoxide as an oxidizing agent is allowed to pass through the channels facing the cathode. This cell is operated at an elevated temperature of, for example, about 650.degree. C.
The cathode and anode both are made of an electron-conductive and porous material such as sponge nickel. The electrolyte is contained in an electrolyte plate, i.e., the plate comprises a matrix of a compacted powdery material such as LiAlO.sub.2 impregnated with a molten carbonate such as potassium carbonate (K.sub.2 CO.sub.3) and lithium carbonate (Li.sub.2 CO.sub.3). The separator is in general made of stainless steel such as SUS 310 or ceramics.
Heretofore there have been proposed various electrodes for such fuel cells, for example, made of a nickel powder-deposited nickel or stainless steel wire net [see Japanese Patent KOKAI (Laid-Open) Nos. 57-34668, 52-136336 and 57-40866].
These prior art electrodes may be plastically deformed with the lapse of time during operation of the cell, or sintered when used in a short time to reduce the specific surface area of a porous electrode, so that the performance of the cell is deteriorated.
In order to overcome such problems as mentioned above, there have been proposed various methods: one in which a nickel oxide powder and a metal oxide powder are mixed, pulverized and pelletized, and the resulting pellets are subjected to the reducing treatment to selectively reduce the nickel and the selectively reduced powder is used as an electrode material, and another one in which a metal element is coated on a ceramic carrier to form an electrode [see U.S. Pat. No. 4,247,604 and Japanese Patent KOKAI (Laid-Open) Nos. 61-271749, 62-2455, 61-267267, 62-5566, 60-253162, 62-76159 and 61-22569].
However, electrodes using these oxide-mixed metal powder, i.e., comprising different layers, a metal layer and an oxide layer inconveniently have cracks generated therein during the process of preparing the electrodes in which the electrodes are repeatedly heated for oxidation and reduction. Furthermore, warp or wave is generated in the electrodes, because the different layers are different in coefficient of thermal expansion from each other. Therefore, the prior art electrodes have such problems that contact resistance between the electrodes and the collectors and/or electrolyte plate is increased. Thus, there have been difficulties in obtaining molten carbonate type fuel cells having a stabilized cell performance.
Moreover, cells using the prior art electrodes have been intended to have a current density of at most 200 mA/cm.sup.2. For example, the electrode according to an embodiment of Japanese Patent KOKAI (Laid-Open) No. 61-22569 referred to above is made by filling a foamed nickel plate as an anode at one side thereof with a paste comprising a mixture of carbonyl nickel and chromium in an ethanol solution of polyvinyl butyral and drying the plate. The paste-filled side faces the electrolyte plate when assembled in a cell. The electrode is not filled at the opposite side with the paste for maintaining the good diffusion of gases. From the cell using the electrode according to Japanese Patent KOKAI (Laid-Open) No. 61-22569 can be taken a current density of at most 200 mA/cm.sup.2.
On the other hand, there has been a demand to obtain a molten carbonate type fuel cell from which so high a current density as about 300 mA/cm.sup.2 can be taken without increasing an amount of internal resistance.